The present invention relates to a smoke generator apparatus and method and especially to the generation of smoke using an electrical resistance heating tube smoke generator.
Although there are a variety of types of smoke generators, only one system is available for firefighter trainers with a flame environment. This system uses a synthetic oil, such as Fyrquel 220 made by AKZO Chemical, Inc., which is the only smoke agent material considered acceptable by the Navy for their fire fighting trainers and more recently for civilian fire fighting trainers. These trainers utilize propane burners which are essentially smoke free and hence have a need for a supplemental machine made smoke for realistic fire fighting training conditions. Another use for high temperature resistance smoke is for periodic leak testing of steam boiler systems that utilize high temperature pressure vessels and piping where cool-down for leak testing is cost prohibitive and can be avoided by using high temperature smoke. So far, smoke agent fluid, such as Fyrquel 220, is the only material considered acceptable for these applications due to its high temperature resistance properties and low toxicity.
This smoke agent fluid, however, requires special design considerations and cannot be used with existing conventional low temperature (500.degree. F. or lower) smoke generators. The reasons are not only the high operating temperatures (1000.degree. F.), but also the tendency of the heated fluid to decompose with high temperature resident heating time, the time that the smoke agent fluid is in contact with heater surface, or the time that an element of fluid is at high temperature, is exceeded and when the heated smoke agent material is exposed to moisture or moist air at high temperatures, such as about 200.degree. F. Hence, it is necessary to avoid or sufficiently minimize stagnant non-flow conditions at high temperatures in start-up and cool-down (shut-off) conditions. The smoke agent fluid is particularly susceptible to moisture contamination during cool-down since the heater tube (or boiling chamber) can have a negative pressure permitting moist air to enter the heating coil.
Only one existing smoke generator is currently able to avoid both high temperature residence time effects and high temperature moisture contamination. This is accomplished by forcing superheated air through a nozzle and then injecting the smoke agent fluid into this high velocity air stream. This method avoids the decomposition/clogging problem but it is expensive due to the high cost of heating and then discarding the air heat source and due to the need for a high pressure air blower. Such a system has a low heating efficiency due to wasted heat of the hot air medium as it exits from the outlet orifice with the smoke generating vapors. This exiting high temperature also tends to interfere with the condensation process required for an effective smoke output.
The conventional "low temperature" smoke generators are not amenable to the use of high temperature smoke agent due to their lengthy warm-up and cool-down times which presents residence heating problems. Another type of smoke generator utilizes a thin wall tube as an electrical resistance heater and has been used successfully for low temperatures of under 550.degree. F. for non-flammable environments. One version of this type of smoke generator requires expensive temperature control considerations due to the low heater mass and long tube length. Either a special fast responding temperature controller that senses resistance changes with temperature distribution along the tube length or a special heat conductive, electrically non-conductive coating is required which permits the use of a point source temperature sensor. The coating, however, also increases the start-up heating time and shut-down cooling time. These two approaches preclude their use as a high temperature smoke generator due to the excessive high temperature residence time during transient start-up and/or shut-down and high temperature moisture contamination during cool-down.
A smoke generator suitable for low temperature smoke generation can be seen in my prior U.S. Pat. No. 4,818,843. This prior smoke generator utilizes a thin wall tube as an electrical resistance heater and utilizes a thin coating over the resistance heating tube which electrically isolates the coils of the coil electrical resistance heating tube while conducting heat through the coils from the hot to the cold end of the coiled electrical resistance heating tube. Other smoke generators can be seen in applicant's U.S. Pat. Nos. 4,547,656 and 4,568,820.
The present invention utilizes a resistance tube heater with a combination of gas injection at the nozzle outlet which atomizes the liquid output during start-up and provides rapid nozzle cooling at shut-down and (2) a high velocity air blower during shut-down for rapid heater cool-down time. This approach permits a continuous pump flow at all phases of operations and cool-down and thus avoids the occurrence of the dangerous hot liquid spray during start-up and lower resident heating time. Thus, the destructive effect of chemical decomposition due to the stagnant type heating is avoided since excessive pump delay as a method of preheating the smoke agent to avoid hot liquid spray would not be required. Also avoided is the stagnant heating of "empty" coils with smoke agent liquid residue coated heater surfaces which tends to breakdown the stagnant layer of liquid agent and eventual clogging of the heater coil.